1. Field of the Invention
This invention relates generally to RF power amplifiers and more particularly to increasing the amplifier's efficiency at low power inputs.
2. Description of the Related
Today's mobile communication systems require high performance transmitters that are able to operate over wide output power ranges and input voltage supply variations without impacting the performance of the system. The proximity of the transmitter with respect to its base station will determine the value of a control signal that correspondingly controls the amount of output power the RF power amplifier must deliver to the antenna. This control signal is normally a pre-programmed range of voltages that correspond to a minimum and maximum operating output power of the power amplifier. Required power levels that fall between the minimum and maximum are typically extrapolated. Also, with mobile communications, the supply voltages tend to change radically with the charge and discharge cycle of the battery. These variations in voltage can adversely effect output power when the voltage is below nominal value and adversely effect the efficiency when voltage is above nominal value.
An existing power amplifier (PA) solution has been implemented using a voltage variable attenuator (VVA) followed by a three-stage, high efficiency, Gallium Arsenide (GaAs) power amplifier. However, when the output power of the PA is reduced by increasing VVA attenuation, which backs off on the drive level, the PA behaves as if it were a class-A amplifier. The operating efficiency of the transmitter is lowered when the amplifier is backed off.
In mobile digital cellular communication systems, the RF output power amplifier consumes a large portion of the battery power budget. A PA with poor efficiency will drain the battery and result in a greatly reduced talk time. Therefore, a constant current PA for the cellular application is undesirable due to the increased power consumption when the PA is backed off. The mobile transmitter should provide low current drain on the battery whenever maximum output power is not required.